NOTES
ON
Power Plant Engineering
ME-602
Unit I: Introduction to methods of converting various energy sources to electric power, direct conversion
methods renewable energy sources, solar, wind, tidal, geothermal, bio-thermal, biogas and hybrid energy
systems, fuel cells, thermoelectric modules, MHD-Converter.
BY:
Ravi Kant Rathore
Asst. Prof.
Mech. Deptt.
GIIT GWALIOR
INTRODUCTION
Two important area of application of thermodynamics are power generation and
refrigeration.
Both power generation and refrigeration are usually accomplished by a system that
operates on a thermodynamics cycle.
Thermodynamics cycles can be divided into two generation categories :
(a)
Power Cycles
(b)
Refrigeration Cycles
The devices or systems used to produce a net power output are often called engines and
the thermodynamics cycles they operate on are called power cycle.
The devices or systems use to produce refrigeration are called refrigerator, air
conditioners or heat pumps and the cycles they operates on are called refrigeration
cycles.
Thermodynamic cycles can be categorized as :
(a)
Power cycles or Refrigeration cycles.
(b)
Gas Cycles or Vapor Cycles : In gas cycles, the working fluid remains in
the gaseous phase throughout the entire cycle, where as in vapor cycles the
working fluid exists in the vapor phase during one part of the cycle and in
the liquid phase during another part.
(c)
Closed Cycles or Open Cycles : In closed cycles, the working fluid is
returned to the initial state at the end of the cycle and is re-circulated. In
open cycle, the working fluid is renewed at the end of each cycle instead of
being re-circulated.

BASIC CONSIDERATION IN THE ANALYSIS OF POWER
CYCLES
Actual Cycle
The cycles encountered in actual devices are difficult to analyze because of the
presence of complicating effects, such as friction and the absence of sufficient
time for establishment of the equilibrium conditions during the cycle.
Ideal Cycle
When the actual cycle is stripped of all the internal irreversibilities and
complexities, we end up with a cycle that resembles the actual cycle closely but is
made up totally of internally reversible processes. Such a cycle is called an Ideal
cycle.
Heat Engines
Heat engines are designed for the purpose of converting other form of
energy to work and their performance is expressed as thermal efficiency.
W

th
net
Qin
The Idealization and Simplification
(a)
The cycle does not involve any friction.
(b)
All expansion and compression process take place in a quasiequilibrium manner.
(c)
The pipe connecting the various component of a system are well
insulated and heat transfer and pressure drop through them are
negligible.
Carnot Cycle
The Carnot cycle is composed of 4 totally reversible processes :
(a)
Isothermal heat addition at high temperature (TH).
(b)
Isentropic expansion from high temperature to low temperature.
(c)
Isothermal heat rejection at low temperature (TL).
(d)
Isentropic compression from low temperature to high temperature.
TL
Thermal efficiency of Carnot cycle = 
1 
th, carnot
TH
The Carnot Vapor Cycle
(a)
A steady-flow Carnot cycle executed with the saturation dome of a pure
substance is shown in Figures 2.1(a) and (b). The fluid is heated reversibly
and isothermally in a boiler (process 1-2), expanded isentropically in a
turbine (process 2-3), condensed reversibly and isothermally in a condenser
(process 3-4) and compressed isentropically by a compressor to the initial
state (process 4-1).
(b)
The Carnot cycle is not a suitable model for vapor power cycle because it
cannot be approximated in practice.
T
2
1
4
3
s
(a)
T
2
1
4
3
s
(b)
Carnot Cycle
Rankine Cycle : The Ideal Cycle for Vapor Power Cycle
(a)
The impracticalities associated with Carnot cycle can be eliminated by
superheating the steam in the boiler and condensing it completely in the
condenser. This cycle that results is the Rankine cycle, which is the ideal
cycle for vapor power plants. The construct of power plant and T-s diagram
is shown in Figures 2.2(a) and (b).
qin
Boiler
3
2
Wturb,out
Turbi
wpump,in
Pump
4 qout
1
Condenser
(a)
T
3
qin
Wturb,out
2
4’
1
qout
wpunp,in
s
(b)
Rankine Cycle
vapor at state 3 enters the turbine, where it expands isentropically and produces work by
rotating the shaft connected to an electric generator. The pressure and the temperature of
the steam drops during this process to the values at state 4, where steam enters the
condenser
Process 4-1
At this state, the steam is usually a saturated liquid-vapor mixture with a
high quality. Steam is condensed at constant pressure in the condenser
which is basically a large heat exchanger, by rejecting heat to a cooling
medium from a lake, or a river. Steam leaves the condenser as saturated
liquid and enters the pump, completing the cycle.
Energy Analysis of the Ideal Rankine Cycle
All four components associated with the Rankine cycle (the pump, boiler, turbine
and condenser) are steady-flow devices, and thus all four processes that make up
the Rankine cycle can be analyzed as steady-flow process.
The steady flow equation per unit mass of steam reduces to
q
(q
in
) (w
out
h h (kJ/kg)
w )
in
out
ei
Pump (q = 0) :
( h h ) v(PP)
w
pump, in
where h h
1
2
1
and v v
f @ p1
1
2
1
v
f @ p1
Boiler (w = 0) :
qin h3 h2
Turbine (q = 0) :
w
turbine, out
( h h )
3
4
Condenser (w = 0)
h h
q
out
4
1
The thermal efficiency of the Rankine cycle is determine from
w

q
th
where
q
net
net
q
in
w q
q
in
out

1
in
ww
out
turbine, out
pump, in
Deviation of Actual Vapor Power Cycle from Idealized Ones
The actual vapor power cycle differs from the ideal Rankine cycle, as a result of
irreversibilites in various components. Fluid friction and heat loss to the
surroundings are the two common sources of irreversibilites.
Fluid friction causes pressure drop in the boiler, the condenser and the piping
between various components. Also the pressure at the turbine inlet is somewhat
lower than that at the boiler exit due to the pressure drop in the connecting pipes.
To compensate for these pressure drops, the water must be pumped to a
sufficiently higher pressure than the ideal cycle. This requires a large pump and
larger work input to the pump
T
IDEAL CYCLE
Pressure drop in
the boiler
Pressure drop
in the pump
3
Irreversibility in
the turbine
2
ACTUAL CYCLE
4
1
Pressure drop in
the condenser
s
(a)
T
3
2a
2s
1
4s
4a
s
(b)
Vapour Power Cycle
The other major source of irreversibility is the heat loss from the steam to the
surrounding as the steam flows through various components.
Pa
rti
cu
la
r importance is the irreversibilites occurring within the pump and the turbine. A
pump requires a greater work input, and a turbine produces a smaller
work output as a result of irreversibilties. Under the ideal condition the flow
through these devices is isentropic.
The deviation of actual pumps and turbine from the isentropic ones can be
accurately accounted by isentropic efficiencies, define as :

p
h h
ws
2s
h2 a h1
wa
h h
w
a
3

T
1

4a

h h
w
s
3
4s
How can We Increase the Efficiency of the Rankine cycle?
Than Rankine cycle efficiency can be increased by increasing average temperature
at which heat is transferred to the working fluid in the boiler or decreasing the
average temperature at which heat is rejected from the working fluid in the
condenser. That is, the average fluid temperature should be as high as possible
during heat addition and as low as possible during heat rejection.
The three ways by which efficiency of the Rankine cycle can be increased are :
(a)
Lowering the condenser pressure (Lowers Tlow, av).
(b)
Superheating the steam to high temperatures (Increases Thigh, av).
(c)
Increasing the boiler pressure (Increases Thigh, av).
Lowering the Condenser Pressure (Lowers Tlow, av)
Steam exists as a saturated mixture in the condenser at the saturation
temperature corresponding to the pressure inside the condenser. Therefore,
lowering the operating pressure of the condenser automatically lower the
temperature of the steam, and thus the temperature at which heat is rejected.
The effect of lowering the condenser pressure on the Rankine cycle
efficiency is illustrated .
T
3
2
2’
4
1
1’
P’4< P4 4’
Increase in wnet
s
Rankine Cycle
Drawback of lowering the condenser pressure is increase in the moisture
content of the steam at the final stages of the turbine. The presence of large
quantities of moisture is highly undesirable in turbines because it decreases
the turbine efficiency and erodes the turbine blades.
Superheating the Steam to High Temperatures (Increases Thigh, av)
The average temperature at which heat is added to the steam can be
increased without increasing the boiler pressure by superheating the steam
to high temperatures. The effect of superheating on the performance of
vapor power cycle is illustrated on a T-s diagram.
Superheating the steam to higher temperatures has very desirable effect : It
decreases the moisture content of the steam at the turbine exit as can be
seen in T-s diagram
The temperature to which steam can be superheated is limited by
metallurgical consideration.
T
Increase in wnet
3’
3
2
1
4 4’
Vapour Power Cycle
T
Increase
in w net
3’ 3
Increase
in wnet
2’
2
1
4’ 4
s
Vapour Power Cycle
Increasing the Boiler Pressure (Increases Thigh, av)
The average temperature during the heat addition process is to increase the
operating pressure of the boiler, which automatically raises the temperature
at which boiling take place. This, in turn, raises the average temperature at
which heat is added to the steam and thus raises the thermal efficiency of
the cycle.
The Ideal Reheat Rankine Cycle
The efficiency of the Rankine cycle can increase by expanding the steam in the
turbine in two stages, and reheating it in between. Reheating is a practical solution
to the excessive moisture problem in turbines, and it is commonly used in modern
steam power plants.
The schematic and T-s diagram of the ideal reheat Rankine cycle.
The ideal reheat Rankine cycle differs from the simple ideal Rankine cycle in that
the expansion process take place in two stages. In first stage (the high-pressure
turbine), steam is expanded isentropically to an intermediate pressure and sent
back to the boiler where it is reheated at constant pressure, usually to the inlet
temperature of the first turbine stage. Steam then expands isentropically in the
second stage (low-pressure turbine) to the condenser pressure.
3
High - p
turbine
Boiler
Low p
trine
Reheater 4
P4 = P5 =
6
5
Condenser
2
Pump
(a)
Reheating
T
High-pressure
turbine
3
4
2’
Low pressure
4
2
1
6
s
(b)
Ideal Reheat Rankine Cycle
Thus the total heat input and the total work output for a reheat cycle become :
q q
q
(h h )
inprimaryreheat
w
w
turbine, out
(hh)
32
5
w
turbine, I
4
(h h )
turbine, II
3
4
(hh)
56
The Ideal Regenerative Rankine Cycle
T-s diagram for the Rankine cycle shows that heat transferred to the working fluid
during process 2-2at a relatively low temperature. This lowers the average heat-addition
temperature and thus the cycle efficiency.
To remedy this shortcoming, the temperature of the liquid leaving the pump
(called feedwater) before it enters the boiler need to be increased.
T
Lowtemperature
heat addition
Steam exiting
boiler
2’
Steam
entering boiler
2
1
4
s
Ideal Regenerative Rankine Cycle
Another way of increasing the thermal efficiency of the Rankine cycle is by
regeneration. During a regeneration process, liquid water (feedwater) leaving the
pump is heated by steam bled off the turbine at some intermediate pressure in
devices called feedwater heaters.
There are two type of feedwater Heaters :
(a)
Open Feedwater Heater
(b)
Closed Feedwater Heater
Open Feedwater Heater
An open (or direct-contact) feedwater heater is basically a mixing chamber,
where the steam extracted from the turbine mixes with the feedwater exiting
the pump. Ideally, the mixture leaves the heater as a saturated liquid at the
heater pressure. The schematic of a steam power plant with one open
feedwater heater and the T-s diagram of the cycle
.
The heat and work interaction of a regenerative Rankine cycle with one
feedwater heater can be expressed per unit mass of steam flowing through
the boiler as follows :
qin h5 h4
q (1 y) (hh)
out
7
1
5
6
( h h )
w
turbine, out
67
(1 y) w
w
turbine, in
y
where
(1 y )(hh )
w
pump I, inpump II, in
m
6
m5
v ( p
w
pump I, in
1
p )
2
v ( p
w
pump II, in
3
1
p )
4
3
5
Turbine
Boiler
y
Open
6-y
1-y
FWH
4
2
Condenser
Pump II
Pump 1
(a)
T
5
4
6
3
3
1
7
s
(b)
Steam Power Plant
The thermal efficiency of the Rankine cycle increases as a result of
regeneration. This is because regeneration raises the average
temperature at which heat is transferred to the steam in the boiler by
raising the temperature of the water before it enters the boiler.
Closed Feedwater Heaters
Another type of feedwater heater used is steam power plants is the closed
feedwater heater in which heat is transferred from the extracted steam to the
feedwater without any mixing taking place. The two streams now can be at
different pressure, since they do not mix. The schematic of a steam power
plant with one closed feedwater heater and the T-s diagram of the cycle
6
Turbine
Boiler
Mixing
chambe
r
7
8
Closed
FWH
9
5
4
2
3
Condenser
Pump 1
Pump II
(a)
T
6
4
5
9
2
7
3
1
8
s
(b)
Steam Power Plant
STEAM GENERATOR
Steam is an important medium of producing mechanical energy. Steam has the advantage
that, it can be raised from water which is available in abundance it does not react much
with the materials of the equipment of power plant and is stable at the temperature
required in the plant. Steam is used to drive steam engines, steam turbines etc. Steam
power station is most suitable where coal is available in abundance.
Thermal electrical power generation is one of the major methods. Out of total power
developed in India about 60% is thermal. For a thermal power plant the range of pressure
may vary from 10 kg/cm2 to super critical pressures and the range of temperature may be
from 250°C to 650°C.
Essentials of Steam Power Plant Equipment
A steam power plant must have following equipment :
(a)
A furnace to burn the fuel.
(b)
Steam generator or boiler containing water. Heat generated in the furnace is utilized to convert water
into steam.
(c)
Main power unit such as an engine or turbine to use the heat energy of steam and perform work.
(d)
Piping system to convey steam and water.
In addition to the above equipment the plant requires various auxiliaries and accessories depending upon the
availability of water, fuel and the service for which the plant is intended.
The flow sheet of a thermal power plant consists of the following four main circuits :
(a)
Feed water and steam flow circuit.
(b)
Coal and ash circuit.
(c)
Air and gas circuit.
(d)
Cooling water circuit.
A steam power plant using steam as working substance works basically on Rankine cycle.
Steam is generated in a boiler, expanded in the prime mover and condensed in the condenser and fed into
the boiler again.
The different types of systems and components used in steam power plant are as follows :
(a)
High pressure boiler
(b)
Prime mover
(c)
Condensers and cooling towers
(d)
Coal handling system
(e)
Ash and dust handling system
(f)
Draught system
(g)
Feed water purification plant
(h)
Pumping system
(i)
Air preheater, economizer, super heater, feed heaters.
shows a schematic arrangement of equipment of a steam power station. Coal received in coal storage yard of power
station is transferred in the furnace by coal handling unit. Heat produced due to burning of coal is utilized in
converting water contained in boiler drum into steam at suitable pressure and temperature. The steam generated is
passed through the superheater. Superheated steam then flows through the turbine. After doing work in the turbine
the pressure of steam is reduced. Steam leaving the turbine passes through the condenser which is maintained the
low pressure of steam at the exhaust of turbine. Steam pressure in the condenser depends upon flow rate and
temperature of cooling water and on effectiveness of air removal equipment. Water circulating through the
condenser may be taken from the various sources such as river, lake or sea. If sufficient quantity of water is not
available the hot water coming out of the condenser may be cooled in cooling towers and circulated again through
the condenser. Bled steam taken from the turbine at suitable extraction points is sent to low pressure and high
pressure water heaters.